1. Field of the Invention
The present invention relates to a power supply circuit of switching regulator type and, more particularly, to an improvement thereof.
2. Description of the Prior Art
A power supply circuit of switching regulator type can be classified into a number of groups, and one of which includes a switching transistor and a converter transformer effecting a blocking oscillation. An example of such a power supply circuit is disclosed in U.S. Pat. No. 4,361,865 of the same applicant as the present application, and is described briefly hereinbelow with reference to FIG. 1.
The prior art power supply circuit of FIG. 1 comprises input rectifying portion 1, blocking oscillator portion 2, converter transformer 3, control circuit portion 5, and output rectifying portion 6. The input rectifying portion 1 receives an AC power from a commercial power line and produces a DC power across capacitor C2. The switching transistor TR4 carries out on-and-off operations repeatedly in a manner described later.
When the transistor TR4 is on, a voltage across the input winding N1 becomes high as shown by a waveform (a) in FIG. 2, and, at the same time, a current Ii flows from one side of the capacitor C2 through input winding N1, the collector-emitter transistor TR4, resistor R14, resistor R11, and junction A to the other side of the capacitor C2, as shown by a waveform (b) in FIG. 2. The resistor R14 is provided for emitter feedback, and the resistor R11 is provided for current detection. Since the current Ii gradually increases, a voltage drop across the resister R11 increases gradually. Thus, the voltage level at a junction A gradually decreases with respect to the voltage level at a line L1. The current Ii flowing through the winding N1 induces voltage across other windings, which are feedback winding NB, detecting winding NC and output winding N2. The voltage induced across the windings NB and N2 are shown by waveforms (c) and (d), respectively, and a current flowing through the winding N2 is shown by a waveform (e). It is to be noted that the voltage across the feedback winding NB has the same polarity as that across the winding N1, but the voltage across the winding N2 has the polarity opposite to that across the winding N1. During the transistor TR4 is on, an energy is stored in the winding N1. Furthermore, during the same period, the voltage produced across the winding NB allows a flow of positive feedback current If through base-emitter of the transistor TR4, resisters R14, positive feedback current control circuit SK (defined by resistors R12 and R13, diode D8 and capacitor C4) and back to the winding NB. Thus, switching transistor TR4 is maintained on.
Then, when the switching transistor TR4 is turned off in a manner described later, a current flows through the winding N2 to dissipate the energy, thereby producing a stable power from the output rectifying portion 6. Furthermore, during the transistor TR4 is turned off, the detecting winding NC produces a voltage which is relative to the voltage from the output rectifying portion 6, whereby during the transistor TR4 being off, the capacitor C3 holds a voltage which is proportional to the voltage produced from the output rectifying portion 6. When the voltage charged across the capacitor C3 is relatively high, i.e., when the power produced from the output rectifying portion 6 is above a standard level, base voltage of the transistor TR1 becomes high to increase the conductivity of the transistor TR1. Thus, a current discharged from the capacitor C3 and fed to the resistor R15 partly bleeds through the transistor TR1 and zenor diode D5 to the line L1, while the remaining current fed to the resister R15 flows through the resistors R7 and R8. When there is a bleeding current through the transistor TR1, the voltage level at the collector of the transistor TR1 becomes low, and, therefore, the voltage level at the junction between the resistors R7 and R8 becomes low. In other words, higher the voltage charged across the capacitor C3, greater the bleeding current and, thus, greater the voltage reduction at the junction between the resistors R7 and R8. At the junction B, voltage from the junction A through the capacitor C7 and resistor R16 is added with the voltage between the resistors R7 and R8. The capacitor C7 and resistor R16 are provided for cutting the DC component and for adjusting the voltage. Thus, the voltage at the junction B varies as shown by a waveform (f).
The on-and-off operation of the transistor TR4 is carried out in the following manner.
When the main switch SW turns on, a starting current Is flows through resistor R2 to the base of switching transistor TR4, whereby the switching transistor TR4 is turned on. During the switching transistor TR4 is turned on, the voltage at the junction A gradually decreases and, therefore, the voltage at a junction B gradually degreases. The junction B is connected to the base of transistor TR2. When the voltage, or potential, at the junction B falls below a predetermined level, the transistor TR2 becomes conductive thereby turning the transistor TR3 conductive. Thus a discharge current flows from capacitor C5 resistor R10, transistor TR3, resistor R14 and transistor TR4 to reverse bias the transistor TR4. Accordingly, the transistor TR4 is turned off. During the transistor TR4 is off, the output rectifying portion 6 produces output power, and at the same time, the capacitor C3 stores a voltage relative to the output power. Furthermore, during transistor TR4 is off, negative feedback current Ir flows from the winding NB through the diode D7 and capacitor C5 to charge the capacitor C5 in the polarity shown in FIG. 1, thus maintaining the transistor TR4 off. Then, by the resonance caused by the inductance of the input winding N1 and a distributed capacitance (a capacitor Cr shown in the drawings represents such a distributed capacitance), the switching transistor TR4 is biased forwardly, thereby turning the transistor TR4 on again.
From the start of current flow through the transistor TR4 in this cycle, the voltage level at the junction A gradually decreases and thus, the voltage level at the junction B gradually decreases. The voltage at the junction B decrease from a level determined by the voltage charged across the capacitor C3. When the voltage charged across the capacitor C3 is high, the voltage at the junction B decreases from a relatively low level and when the voltage charged across the capacitor C3 is low, the voltage at the junction B decreases from a relatively high level. Then, when the voltage at the junction B decreases to the predetermined level, the transistor TR2 conducts to turn the switching transistor TR4 off. Thus, a period in which the switching transistor TR4 is on is determined by the voltage level charged in the capacitor C3. During the transistor TR4 is on, energy is accumulated in the transformer 3, and when it is off, the accumulated energy is dissipated by the current flow from the output rectifying portion 6.
By controlling the duration of on-period T1 (FIG. 2, row (g)) of the switching transistor TR4, the power produced from the output rectifying portion 6 can be maintained constant regardless of unwanted fluctuation in the input AC power and of fluctuation in the load.
The prior art power supply circuit has following disadvantages.
Fist of all, the prior art power supply circuit has a poor stability of the output current. In the above described power supply circuit, a current flowing through a load (not shown) connected to the output rectifying portion 6 increases in a following manner.
Since the power consumed in the load is proportional to the energy accumulated in the winding N1 by the current Ii during the on-period T1 of the switching transistor TR4, the increase of load current results in increase of peak value Icp of the current Ii. More specifically, when the load current increases, the DC voltage between the lines L1 and L2 becomes low and, therefore, the voltage level (level lo in FIG. 2 waveform (f)) at the junction B becomes high. Thus, the moment when the transistor TR2 is turned on is delayed, thereby prolonging the duration of on-period T1 of the switching transistor TR4. Since the current Ii increases incessantly during the on-period T1 of the transistor TR4, the peak value Icp of the current Ii becomes high. Therefore, according to the prior art power supply circuit shown in FIG. 1, the maximum available load current which can be produced from the output rectifying portion 6 is accomplished when the transistor TR1 is held off to provide a maximum available voltage to the junction B, thereby maintaining the switching transistor TR4 on for the maximum available period.
According to the prior art power supply circuit, the peak value Icp of the current Ii is dependent not only to the on-period T1 as described above, but also it is dependent on input voltage Vi across the capacitor C2, as indicated below: EQU Icp=Vi.times.T1/L1
wherein L1 is an inductance of the input winding N1. From the equation, it is understood that the peak Icp of the input current Ii becomes high as the input voltage Vi becomes high. In other word, the maximum available output current varies with respect to the change of the input voltage Vi, as shown by output current to output voltage characteristic curve in FIG. 4a, wherein a solid line represents a case where the input voltage Vi is at the standard level, dotted line represents a case where the input voltage Vi is below the standard level, and a dot-dash line represents a case where the input voltage Vi is above the standard level. In the case where the input voltage Vi is below the standard level, the maximum available current is below the standard level and, where the input voltage Vi is above the standard level, the maximum available current is above the standard level.
Thus, the prior art power supply circuit has a poor stability in output current. Therefore, the prior art power supply circuit is not suitable for use, for example, in a television receiver, because in the television receiver, since a current flowing through the CRT (cathode ray tube) varies greatly with respect to the change of brightness of the picture on the CRT, a total required current flowing through the television receiver also changes greatly. From this view point, it is necessary to enable the power supply circuit to supply a current sufficient to cope with the maximum current required by the television receiver even when the maximum available current from the power supply circuit is low due to the low input voltage Vi. This means that, it is necessary to design the power supply circuit or each circuit in the television receiver in such a manner as to render the maximum available current greater than the maximum required current even when the input voltage Vi is comparatively low, resulting in complicated structure and less freedom on design.
The second disadvantage is in the transistor TR2. During the off-period of the switching transistor TR4, the transistor TR4 is reverse biased by the voltage generated across the feedback winding NB and, at this moment, the capacitor C5 is charged with a voltage which is slightly lower than the voltage from the feedback winding NB. Therefore, the voltage level at the junction D, which is identical to the emitter of the transistor TR2, is almost the same as the voltage level at the line L1. Since no negative voltage, under this condition, will be supplied from the junction A to the junction B, the voltage level at the junction B is made sufficiently high with respect to the voltage level at the line L1. Therefore, the transistor TR2 is reverse biased between its base and emitter with a high voltage. Thus, the prior art power supply circuit requires a transistor TR2 having a high dielectric strength with respect to the voltage applied reversely between the base and emitter thereof.
The third disadvantage is in the arrangement of converter transformer 3. According to the prior art power supply circuit, the feedback winding NB and the detecting winding NC are provided separately; and a circuit (carrying current Ir) for charging the capacitor C5 is electrically floating with respect to the line L1. Therefore, it is necessary to provide a high electric insulation between the windings NB and NC, resulting in bulky size and complicated structure of the converter transformer 3. Moreover, its manufacturing cost increases.
The fourth disadvantage is in the arrangement for providing a biasing voltage to the transistor TR2. According to the prior art power supply circuit, the transistor TR2 is provided with biasing voltage from the junction A through resistor R16 and capacitor C7, in addition to the voltage from the junction between the resistors R7 and R8. Since the capacitor C7 is necessary for cutting the DC component, and the resistor R16 is necessary for regulating the voltage, the circuit elements C7 and R16 can not be eliminated.
The fifth disadvantage is in the arrangement of the positive feedback current control circuit SK. According to the prior art power supply circuit mentioned above, the positive feedback current If flowing through the circuit SK varies relatively to the change of input voltage Vi, causing following drawbacks. As described above, the positive feedback current If flows through the circuit SK, and mostly through the resistor R13 and diode D8, because the impedance of the capacitor C4 and resistor R12 is very high when compared with that of the resistor R13. In this case, since the impedance of the resistor R13 and diode D8 can be considered as constant, the positive feedback current If can be considered as being proportional to the voltage developed across the winding NB during the on-period of the switching transistor TR4, and thus, it is proportional to the voltage Vi provided across the input winding N1. Therefore, the positive feedback current If becomes great as the input voltage Vi becomes high, and vice versa.
In the case where the input voltage Vi is high, the on-period of the switching transistor TR4 becomes short and, therefore, the peak value Icp of the current Ii becomes rather low. Therefore, in spite of low level of the current Ii, a large amount of positive feedback current If flows through the base of the transistor TR4. Thus, the switching transistor TR4 operates beyond the rated range, resulting in over-drive operation.
On the other hand, when the input voltage Vi is rather low, the on-period of the switching transistor TR4 becomes long and, therefore, the peak value Icp of the current Ii becomes rather high. Therefore, in spite of high level of the current Ii, the positive feedback current If is small. In this case, the switching transistor TR4 is driven in a so-called under-drive condition.
Thus, in the case when the input voltage fluctuates to a relatively low or high level, the collector loss of the transistor TR4 becomes great and, thus, the range in which the power supply circuit operates stable becomes narrow.
Furthermore, although the turn-off operation of the switching transistor TR4 is carried out by the reverse bias current Id flowing from the charged capacitor C3, the current Id remains constant regardless of fluctuation in the input voltage Vi. This is because the capacitor C3 is charged during off-period of the transistor TR4 by the voltage generated across the winding NB, and such a voltage is controlled to be constant under the stable operating condition. In the case where the input voltage is relatively high, however, the switching transistor TR4 is in the over-drive condition and, thus, the reverse bias current Id may fail to turn-off the switching transistor TR4, because of lack of current level in the reverse bias current Id. From this view point also, the stable operating range of the power supply circuit becomes narrow.
In addition to the above, since the capacitor C5 is not charged when the main switch SW is turned on, the collector current Ii of the transistor TR4 reaches as high as .beta. times the positive feedback current If. Therefore, in the case where the input voltage Vi is high, the positive feedback current If becomes high. Thus, the collector current Ii increases above the rated range, or safety range, thereby causing damage to the transistor TR4.
Furthermore, in the case of over-loading, the increase of load current affects on the secondary winding N2 to reduce the output voltage. On the other hand, however, the collector current Ii of the transistor TR4 increases so as to prevent such a voltage drop down in the secondary winding N2. In this case, if the input voltage is rather high, it is possible to supply a large amount of positive feedback current If to the transistor TR4. If this happens, the collector current Ii also increases, causing damage to the transistor TR4.